This invention relates to a mounting arrangement for a noise canceling microphone, and more particularly to a mounting arrangement for a higher order microphone within a small terminal such as a handset, portable terminal, or a neckset.
Modern microphones for use in speech terminals are required to provide improved transmission characteristics. There are essentially two types of microphones widely used in telephony, namely pressure microphones, or zeroth order microphones, and pressure gradient microphones, or first order microphones.
Microphones higher than zeroth order are used to provide some immunity to acoustic noise. These microphones can discriminate sounds both by source direction and source proximity.
Pressure microphones have an omnidirectional directivity pattern, while pressure gradient microphones can have directivity patterns such as cardioid, which are most sensitive to sounds impinging from in front of the microphone, decreasing as the direction approaches the back, or xe2x80x9cFIG. 8xe2x80x9d, which are most sensitive to sounds impinging from the front and back, and least sensitive to sounds impinging from sides. These types of noise canceling microphones need to have access to the sound field in more than one place, requiring two or more ports. A first order microphone is made to be sensitive to a combination of the sum of, and difference between the pressure at two ports. Second and higher order microphones require three or more ports, and the sensitivity is proportional to a combination of the sum of, and the difference between the pressure at these ports.
Ideally, such microphone assemblies (capsules) should operate in a substantially free-field environment in order to have the best noise-canceling effect. This presents a problem in mounting the capsule in a telephone housing due to the increased acoustic impedance presented to the ports, and to diffraction. These effects generally degrade the noise canceling performance of the microphone and alter the frequency response. The following requirements should be taken into consideration to circumvent these effects:
1. diffraction around and reflection from the terminal should not substantially alter the acoustic characteristics of the noise (i.e. planar nature of the wavefronts, and the effective impinging direction);
2. the ports in the housing should be short;
3. the acoustic impedance of the ports in the housing should be much less than the acoustic input impedance of the microphone; and
4. the ports in the housing need to have a high degree of acoustic symmetry.
In modern telephone handsets, the microphone ports are often at the front and back, which usually violates requirements 1 and 4. Some known architectures use a flap for accommodating the microphone. When port locations are the front and back of the flap, as is usually the case, requirements 1 and 4 are again violated.
In most mounting schemes, it is difficult to satisfy requirement 3 because it usually requires careful sealing of the microphone to the housing. This is difficult and expensive in a high volume manufacturing environment. Requirement 2 is usually difficult to meet if substantially electrostatic discharge immunity is needed.
Objects of the Invention
It is an object of this invention to effectively use a noise canceling microphone in a terminal to substantially reduce background noise without adversely affecting the transmitted voice signal.
Another object of the present invention is to provide a mounting arrangement for a noise canceling apparatus that is compact in structure and simple in construction so that it can readily be implemented in a standard telephone handset, a wireless terminal, a neckset or the like.
It is another object of this invention to provide a mounting arrangement for an embedded microphone which complies with the above four requirements.
Briefly stated, according to one aspect, the invention comprises a mounting arrangement for a telephone handset and the like, the handset having a transmit end for housing a capsule with an electroacoustic transducer and two ports, the arrangement comprising walls defining a regular-shaped cavity at the transmit end, and means for fixing the capsule into the cavity with the geometrical center of the capsule substantially corresponding with the geometrical center of the cavity and leaving substantially equal free space around each of the ports.
According to another aspect, the invention comprises a method for mounting a capsule with an electroacoustic transducer into a housing of a telephone handset and the like, comprising the steps of selecting a capsule with two ports, a longitudinal size and a transversal size, providing a regular-shaped cavity at the transmit end of the telephone handset with a depth equal or higher than the transversal size of said capsule, and fixing the capsule into the cavity with the geometrical center of the capsule substantially corresponding with the geometrical center of the cavity and leaving substantially equal free space around each of the ports.
Advantageously, the embedded microphone of the present invention has demonstrable, effective noise reduction characteristics.
Another advantage of the present invention is that it presents a solution for placement of the microphone that is simple, inexpensive and can be implemented in most types of voice terminals.